(a) Technical Field
The present invention relates to a humidifier for a fuel cell. More particularly, it relates to a humidifier, which is used as an auxiliary humidifier of a fuel cell together with an existing gas-to-gas humidifier. The humidifier, when used as an auxiliary humidifier, can provide improved humidification performance in a low efficiency region of the gas-to-gas humidifier.
(b) Background Art
A fuel cell is an electricity generation system that does not convert chemical energy of fuel into heat by combustion, but rather electrochemically converts the chemical energy directly into electrical energy in a fuel cell stack. The fuel cell can be applied to the electric power supply of small-sized electrical and electronic devices, for example portable devices, as well as industrial and household appliances and vehicles.
One of the most widely used fuel cells for a vehicle is a proton exchange membrane fuel cell or a polymer electrolyte membrane fuel cell (PEMFC), which includes a fuel cell stack comprising a membrane electrode assembly (MEA), a gas diffusion layer (GDL), a gasket, a sealing member, and a bipolar plate (separator). Generally, the MEA includes a polymer electrolyte membrane and an electrode/catalyst layer disposed on each of both sides of the polymer electrolyte membrane. Hydrogen ions are transported through the polymer electrolyte membrane and an electrochemical reaction takes place in the electrode/catalyst layer. The GDL functions to uniformly diffuse reactant gases and transmit generated electricity. The gasket functions to provide an appropriate airtightness to reactant gases and coolant. The sealing member functions to provide an appropriate bonding pressure. The bipolar plate functions to support the MEA and GDL, collect and transmit generated electricity, transmit reactant gases, transmit and remove reaction products, and transmit coolant to remove reaction heat, etc.
In the case of a PEMFC used in a fuel cell vehicle, water is required for its operation. Since the water serves as a transport medium for hydrogen ions (H+), the humidity of reactant gases is directly related to the performance of the fuel cell. Therefore, the air (or oxygen) as an oxidant supplied to a cathode of the fuel cell stack is humidified at an air inlet of the fuel cell stack by a humidifier.
If the amount of water is insufficient in the PEMFC, the conductivity of the hydrogen ions is reduced, and further the contact resistance between the electrode and the electrolyte membrane is increased by the contraction of the electrolyte membrane. On the other hand, if the amount of water is in excess, a flooding phenomenon, in which water forms on the electrode, occurs to prevent the diffusion of reactant gases, thereby reducing the performance of the fuel cell. Therefore, proper humidification is required to prevent the flooding phenomenon.
The reactant gases supplied to the fuel cell stack are air and hydrogen, and the air is humidified to a necessary extent by the humidifier before it is supplied to the fuel cell stack. The humidified air is introduced into the cathode of the fuel cell stack through an intake manifold to humidify the membrane electrode assembly to increase the ion conductivity, and is then discharged to the air or condensed air through an exhaust manifold.
There are many types of humidifiers such as a bubble-type humidifier, an injection-type humidifier, an absorbent humidifier, etc. However, in the case of the fuel cell vehicle, a membrane humidifier having a relatively low volume is widely used due to limitations in terms of packaging.
The membrane humidifier has some significant advantages in that no separate power is required as well as packaging. Moreover, in the membrane humidifier, the gas supplied to a cathode inlet of the fuel cell stack can receive the exhaust heat and water of the fuel cell stack from hot and humid gas discharged from a cathode outlet.
FIG. 1 is a schematic diagram showing a state in which the air is humidified by a membrane humidifier 120 and supplied to a fuel cell stack 130 in a typical fuel cell system. As shown FIG. 1, the outside dry air is forcibly blown by an air blower 110 to pass through the membrane humidifier 120.
Here, supersaturated humid air discharged from the cathode outlet of the fuel cell stack 130 is passed through the membrane humidifier 120 to humidify the dry air by water exchange between the supersaturated humid air and the dry air, and the humidified air is supplied to the cathode inlet of the fuel cell stack 130.
The typical membrane humidifier is a gas-to-gas humidifier using a hollow fiber membrane, in which the hollow fiber membrane having a large contact surface area can be highly concentrated. Therefore, the fuel cell can be sufficiently humidified by a small capacity membrane humidifier. Moreover, the water and heat contained in the hot gas discharged from the cathode of the fuel cell stack can be collected and reused by the membrane humidifier, thus saving water and energy used in the humidification.
Various fuel cell systems including an auxiliary humidifier as well as the membrane humidifier to improve the humidification performance and the output performance of the fuel cell stack are described.
Korean Patent Publication No. 10-2008-0042614 describes a hybrid humidification system which comprises an outlet formed in a membrane humidifier such that condensed water discharged through the outlet of the membrane humidifier is stored in a water reservoir and can be injected into the air supplied to a fuel cell stack by an injection pump and an injector when a high output of the fuel cell stack is required, thus increasing the amount of humid air.
U.S. Pat. No. 6,696,192 describes a fuel cell system comprising a water connecting apparatus and an auxiliary humidifier in addition to a membrane humidifier. The water collecting apparatus comprises a vapor-liquid separator and a water storage tank. The auxiliary humidifier comprises a water pump, a check valve, a pipe, and an injector. Water collected in the water storage tank is atomized by the injector and then injected into a fuel cell stack.
As such, the technique of separating water from the exhaust gas and injecting an appropriate amount of water into the supplied gas using the injector is used to improve the humidification performance when the amount of heat and water is not enough during high power operation or during start-up of the fuel cell.
U.S. Pat. No. 7,258,937 describes a fluid flow system comprising an air supply compressor, a vapor-liquid separator, a metering device, an injector, and a controller. The compressor sucks and compresses a mixture of fresh air and humid exhaust gas from a fuel cell stack and supplies the mixture to the fuel cell stack, and the injector injects water separated from the exhaust gas into the mixture in the compressor.
The water injected into the mixture is vaporized by the compression heat of the air to cool the compressor and, at the same time, humidifies the air. It is difficult to sufficiently humidify the air supplied to the fuel cell stack by directly injecting water into the compressor, and thus only a required amount of water is directly injected into the mixture of fresh air and exhaust gas to prevent the reduction in efficiency of the fuel cell system.
However, the above-described conventional humidification systems have the following problems.
(1) The gas-to-gas humidifier is a non-powered humidifier which can collect and reuse the exhaust heat and water from the fuel cell stack at the same time, and thus its efficiency is high. However, since it employs manual humidification, a low efficiency is expected due to the structural properties.
(2) In the case where the injection technique is employed to overcome the low efficiency of the existing gas-to-gas humidifier, the system is complicated by auxiliary devices such as a water reservoir for injection, vapor-liquid separator, injector, injector chamber, pipe, controller, injection volume measurement device, sensors, etc. Thus, the volume, weight, manufacturing cost, noise, etc., are increased. Moreover, there are difficulties in determining the appropriate injection time and amount of the injector in the humidification system comprising the vapor-liquid separator (i.e., water separator), high pressure compression, and injector injection.
(3) To facilitate the humidification, it is necessary to inject microdroplets, and thus it is necessary to increase the injection pressure, use a high pressure pump, ensure the capacity of the water tank, and control the water level. Moreover, in the case where the capacity of the vapor-liquid separator is increased, it is disadvantageous in terms of packaging, the cooling efficiency is unsatisfactory, and thus the deterioration in cooling performance of the fuel cell vehicle is further increased.
(4) To prevent freezing of the water reservoir when the air temperature is below 0° C. such as in winter, antifreezing means such as heating and insulation is required, which causes a delay in start-up due to the heating and increases the power consumption of a battery.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.